GoToSocial servers don't support scheduled posts; they return the wrong
type, and this can cause a loop of posting.
The GoToSocial bug to implement scheduled posts is
https://github.com/superseriousbusiness/gotosocial/issues/1006.
Fix this by adding a new server capability for scheduled post support,
using it for most servers, and disabling it for GoToSocial.
If scheduled post support is not available for an account:
- The "Scheduled posts" menu option is not shown.
- The scheduling button (clock) when composing a post is hidden, so the
user cannot set scheduling parameters.
Fixes#963
Previous code didn't encode v2 filter keywords, so created v2 filters by
first creating the filter with no keywords (one API call) then making
1-N API calls to add each keyword to the filter.
Fix this by adding a dedicated converter for the `NewContentFilter` type
that encodes it correctly so the filter can be created with a single API
call.
This necessitates moving some types around,
`TabData` recorded the type of the timeline the user had added to a tab.
`TimelineKind` is another type that records general information about
configured timelines, with identical properties.
There's no need for both, so remove `TabData` and use `TimelineKind` in
its place.
`TimelineKind` is itself mis-named; it's not just the timeline's kind
but also holds data necessary to display that timeline (e.g., the list
ID if it's a `.UserList`, or the hashtags if it's a `.Hashtags`) so
rename to `Timeline` to better reflect its usage. Move it to a new
`core.model` module.
