Edx-opaque-keys

* Added OpaqueKeyField and its subclasses, extracted from edx-platform
* Added the "django" installation option to indicate the optional dependency
on the Django package

* Stop an assortment of deprecation warnings by replacing internal usage of
deprecated properties.
* Trailing '/' characters are no longer valid in serialized usage keys.

* Enforce the property that serialized keys are equal if and only if the parsed
keys are equal. In particular:
* OpaqueKey parsing is now case-sensitive
* Newlines aren't allowed at the end of any keys
* AssetLocators are correctly terminated by their final non-matching character
For example, '/c4x/-/-/-/-0/c4x' no longer parses to the same key as
'/c4x/-/-/-/-0'
* Stricter requirements for '/' characters in AssetLocators and deprecated
BlockUsageLocators. For instance, 'i4x:/...' and 'c4x/...' are no longer
valid.
* Removed the '/i4x/...' form of deprecated usage keys.
* Fixed improper parsing where misspellings like '0i4x://...' were accepted.
* Added a new version of XBlock Aside keys that encodes all occurances of ':' and '$',
and changed the old version to raise an error when a key would have an
ambiguous encoding.

Newly invalid keys can be found in the tracking logs by using the following
regular expressions (in unicode-aware mode):

grep -E '/c4x/[^/"]+/[^/"]+/[^/"]+/[^"]+([^/"]+)?/' tracking.log unterminated AssetLocators
grep --ignore-case "version" tracking.log | grep -v "version" Mixed case 'version'
grep --ignore-case "branch" tracking.log | grep -v "branch" Mixed case 'branch'
grep --ignore-case "[ic]4x:;_;_" tracking.log | grep -v "[ic]4x:;_;_" Mixed case 'i4x' and 'c4x'
grep --ignore-case "[ic]4x:%2F%2F" tracking.log | grep -v "[ic]4x:%2F%2F" Mixed case 'i4x' and 'c4x'
grep --ignore-case "[ic]4x://" tracking.log | grep -v "[ic]4x://" Mixed case 'i4x' and 'c4x'
grep --ignore-case "(;_)?[ic]4x(;_)" tracking.log | grep -v "(;_)?[ic]4x(;_)" Mixed case 'i4x' and 'c4x'
grep --ignore-case "(%2F)?[ic]4x(%2F)" tracking.log | grep -v "(%2F)?[ic]4x(%2F)" Mixed case 'i4x' and 'c4x'
grep --ignore-case "/?[ic]4x/" tracking.log | grep -v "/?[ic]4x/" Mixed case 'i4x' and 'c4x'
grep -E '"event_type": "[^"]*(%0A|%0D|\\n|\\r)' tracking.log Encoded newlines
grep -P 'i4x:;_(?!;_)|(?<!;_)c4x' tracking.log Missing encoded '/' characters
grep -P 'i4x:%2F(?!%2F)|(?<!%2F)c4x' tracking.log Missing encoded '/' characters
grep -P 'i4x:/[^/]|(?<!/)c4x' tracking.log Missing '/' characters
grep '(;_)?i4x;_' tracking.log Invalid form of i4x://
grep ';_?i4x;_' tracking.log Invalid form of i4x://
grep '%2F?i4x%2F' tracking.log Invalid form of i4x://
grep '/?i4x/' tracking.log Invalid form of i4x://
grep -P '\wi4x:;_' tracking.log i4x:// misspellings
grep -P '\wi4x:%2F' tracking.log i4x:// misspellings
grep -P '\wi4x:/' tracking.log i4x:// misspellings

* Update the regular expression for a course key and locators which use course
keys so that a string with a trailing newline will no longer be accepted as a
valid key.

* Revert of caching optmizations introduced in 0.3.2, due to a bug that can
occur where course keys can be parsed with trailing newlines, and those parsed
values can be serialized into the database.

* Simple optimizations to reduce the number of OpaqueKey objects
created, and to speed up hashing and equality checks.

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-No changelog was maintained before 0.3.2.