Geokey-sapelli

- Fix in `LoginAPI` error message
- `sapelli_project_list.html`template: improvements, fixes & clean-ups in js code dealing with the QR code dialogue
- `SAPDownloadQRLink` model: improved exception handling and added documentation

- Fix for Sapelli projects uploaded prior to v0.6.1: these may have an invalid contribution permission setting

- Bugfix: correctly set contribution permissions for new Sapelli projects
- SapelliItem: removed image field and instead rely on symbol field in categories.LookupValue (new in https://github.com/ExCiteS/geokey/releases/tag/0.8.5)
- LoginAPI: improved error handling
- new helper method to check if extension is correctly installed, if there are problems there are reported on the Sapello project list page
- Massive improvements to QR link functionality:
- QR links now remain valid for 1 day independent of whether the creating user logs out.
- while the QR link is valid it will be shown again (rather than a newly generated one) if the same user asks for it (for the same project)
- the remaining validity time is shown in the QR code dialog
- the creating user can now revoke the QR link at any time before it expires automatically
- all this also applies to hyperlink on the QR image (i.e. it can be distributed to others just like the QR image itself, and will remain valid and expire/be revoked together)
- misc. cleanups

- _Improved_: project-specific menu in admin pages
- _Improved_: safer project loading code (delete any failing SAP file)
- _Improved_: improved test cases/factories
- _New_ **SAPDownloadAPI**: allows to download the SAP file of SapelliProjects, this is exposed through the admin pages.
- _New_ **SAPDownloadQRLinkAPI**: generates a QR code (as a PNG image) which encodes a link to the SAP download for a given project. The image can be shown in the Sapelli project list admin page. The link in the QR code includes the authentication token of the user for whom the QR code is generated (i.e. he/she who is logged into the admin pages). This means other people (or devices!) can scan the code to download the SAP file, but only until the creating user's session expires or he/she logs out of the admin pages.

- new recommended location to place the Sapelli jar file (which does no longer require setting the path in settings.py, although that is still possible)
- made sapelli_loader.py methods more robust (notably with regards to finding the jar file and creating the sapelli working directory)
- clarifications in README.rst

Fixes issues 4, 6, 7, 13, 14, 15, 16 & 17; as well as a few other (potential) problems I didn't bother to create issues for.

The main fundamental change it geokey_sapelli now uses the actual Sapellli java code for the complete process of loading and parsing Sapelli projects from SAP files (issue 17).
This requires this JAR file: https://github.com/ExCiteS/Sapelli/releases/tag/v2.0.0-beta-16.5
Using the Java code for project loading/parsing has several advantages. It means we no longer have to maintain a separate Python implementation of the Sapelli XML format parser. This resolves a number of known issues (such as 6 and 7) and also makes it easier to add support for new or existing Sapelli features to geokey-sapelli.