Tiny improvements: UI, Docs, Overlay Settings, File Upload

[m6121]

This PR proposes several tiny improvements:

• List of projects now evaluates the permissions of the user and displays buttons for update and deletion only if the corresponding permission is available (tests need to be adjusted as they were previously evaluating the update-link instead of the existence of the project)
• If a custom overlay list is configured, the overlay function evaluating the overlay set might produce errors if overlays already are not present and the function tries to remove them
• Minor corrections to dev docs
• Add filter for file uploads that pre-selects file type "xml" as suggested in Layout improvements for discussion #254 no. 16

Looking forward to your thoughts on these changes.

[coveralls]

Coverage remained the same at 91.55% when pulling afb42fd on m6121:template-changes into c8a1794 on rdmorganiser:dev.

[jochenklar]

Cool, thanks.

One minor thing: technically people can create import plugins for non-xml files, so I would not add that.

I will create a dev branch for the new version soonish or we could merge that to master and use it for RDMO 1.6.3 (I already found some other bugs).

[m6121]

Thanks for your feedback. I reverted the file type filter.

The merging options sound both good to me.