Eprints2bags

* Now stores login & password on a per-server basis, instead of (as previously) a single login & password for all servers
* Accepts empty user names and passwords for EPrints servers that don't need them
* Fixed handling lack of `official_url` elements in EPrints records
* Changed how thumbnail images and other files are identified for a given record, by looking at the `<relation>` element to see if it is `isVolatileVersionOf`
* Make sure to write files in UTF-8 format regardless of whether the user's environment variables are set properly. (Previously, having set `LC_ALL` to an unusual value would result in an error such as `'ascii' codec can't encode character '\u2019' in position 3540: ordinal not in range(128)`.)
* Refactor credentials-handling code and remove no-longer-needed `credentials.py`
* Other minor internal changes

* Fixed an important network handling bug that could cause incomplete records to be saved
* Fixed bugs in handling network exceptions while downloading content from servers
* Improved detection of file system limitations
* Makes `-o` an optional argument
* Fixed a missing Python package import
* Renamed `CONDUCT.md` to [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md) so that GitHub can find it
* Added [`CONTRIBUTING.md`](CONTRIBUTING.md),
* Updated the documentation
* Fixed some other minor bugs
* Minor internal code refactoring

Eprints2bags now generates uncompressed [ZIP](https://www.loc.gov/preservation/digital/formats/fdd/fdd000354.shtml) archives of bags by default, instead of using compressed [tar](https://en.wikipedia.org/wiki/Tar_(computing)) format. This was done in the belief that ZIP format is more widely supported and because compressed archive file contents may be more difficult to recover if the archive file becomes corrupted. Also, the program `eprints2bags` now uses the run-time environment's keychain/keyring services to store the user name and password between runs, for convenience when running the program repeatedly. Finally, some of the the command-line options have been changed.