specify JSON-requesty textile-endpoint

- request is issued as JSON
- optional context is provided inside request-body
- response is HTML
specify plain textile-endpoint

- requests as plain-text
-> feels cleaner, needs no escaping
- context is encoded in the URL
-> allows to set context-sensitive links to the textile-endpoint from
within the API
unify property-names around textilzed attributes
mark textileDescription as writable
fix Aglio-warnings
specify supported rendering contexts
choose better examples
more sophisticated formatables

- formatables are real objects
- higher extensibility
- only one formatting supported per property

[ci skip]
generalize rendering endpoints

As long as OpenProject supports "plain" formatting we need to support it
in the API too

[ci skip]
Merge pull request #2094 from opf/feature/specify_patchable_description

[API] Specify endpoints for change of WP-Description (incl. Preview)