unify wording for times and dates

- Types: Duration, Date, DateTime
- always use createdAt, updatedAt (not On etc.)

[ci skip]
mark start and due date writable

[ci skip]
add missing ""

[ci skip]
Add autocomplete helper
Add WP autocomplete path
Set WP autocompletion path
Add autocompletion to activity comment
Add autocompletion to user activity
Add autocompletion to WP description
Update jquery.atwho dependency
Merge pull request #2216 from opf/fix/17334_autocompletion_in_description_and_comments

17334 autocompletion in description and comments
Merge pull request #2234 from opf/feature/specify_patchable_dates

Specify patchable dates
clean tables

- removed examples
- ISO 8601 information moved to "basic objects" section, also linking to Wikipedia

[ci skip]
some more minor cleanup

[ci skip]
Test that collections adhere to the specifiction
Implement collections specification
unify wording to x > 0

[ci skip]
Remove obsolete available statuses endpoint

With the availibility of those statuses in the WP form this became obsolete.
formatting
Merge pull request #2237 from opf/feature/clean_spec_tables

Cleanup API specification tables