enhance media file slug handling

- allow forward slashes in slugs
- increase UI consistency when validating slugs

issue #133
issue #446
rename method to avoid constructor confusion

fixes #475
merge system report upgrades from networks branch
system report: improve error/notice rendering
system report: add notice for upcoming PHP upgrade
update readme
:shipit: release 1.10.18
fix template caching

Since the introduction of an expiry date, the effective cache key got
shorter. This fixes it.
fix error when creating a new episode

fixes #479
close #480
:shipit: release 1.10.19
repair button: fix duplicate contributions

fixes #253
improve contribution counting

only count each contributor once per episode, even he legitimately has
multiple contributions

issue #253
refactor: rename function
refactor: move social repair into social module
refactor: move contribution repair into its module
Merge branch 'release-1-10-20' into changeable_dashboard