Allow creating archived bookmark through REST API

[kencx]

This PR implements a POST method for the /api/bookmarks/archived endpoint that creates an archived bookmark directly.

It would be helpful for #243.

[kencx]

I added the optional is_archived flag to the existing POST endpoint and also ensured that the test verifies all fields.
I will also update the docs after the code has been reviewed, to prevent unnecessary changes.

One problem I foresee is that POST-ing an existing normal bookmark with the is_archived: True flag will archive it, but the reverse is not true, ie. an existing archived bookmark will not be unarchived with the is_archived: False flag.

Of course, there is the archive and unarchive endpoint for this but I feel it makes sense to standardize this behaviour. If you feel the same, I can add this.

[sissbruecker]

One problem I foresee is that POST-ing an existing normal bookmark with the is_archived: True flag will archive it, but the reverse is not true, ie. an existing archived bookmark will not be unarchived with the is_archived: False flag.

With the current changes, the is_archived flag will not be considered at all on updates, since it is not used in the BookmarkSerializer.update method. Overwriting a bookmark's is_archived flag in there could lead to unarchiving an archived bookmark if the is_archived flag is not specified in the payload, which doesn't feel right. I think it's OK to exclude updates from this PR and rely on having the archive / unarchive endpoints for now.

[kencx]

All proposed changes made and I have updated the docs too.

[sissbruecker]

Looks good, updated the API docs a bit