[bugfix] Store and expose status content type (#3870)

* Add ContentType to internal models

* Add ContentType to API models StatusSource and StatusEdit

* Add helpers to convert between API/internal StatusContentType

* Write status content type on create/edit

* Add migration

* Update API docs

go run github.com/go-swagger/go-swagger/cmd/swagger generate spec --scan-models --exclude-deps --output docs/api/swagger.yaml

* ensure ContentType is updated anywhere Text is

* Update docs, take care of TODOs

* Set ContentType in more places where Text is set

* We don't actually use ContentType on the API status model

* Update StatusSource test

* Remove unused helper function I copied

* Revert change to StatusContentType swagger annotation

I'm going to include this in a follow-on PR instead.

* Add test for updating content type in edits

* Return a value from processContentType instead of modifying the existing status

Fixes an issue that was caught by the test I just added - the recorded edit would be marked with the *new* content type instead of the old one, which is obviously bad

* Add test for handling of statuses with no stored content type

* repurpose an existing test status instead of adding a new one to avoid breaking other tests

* Add test to ensure newly created statuses always have content type saved

* Do include content type on status API model actually

This is mostly important when deleting and redrafting.

The comment on `apimodel.Status.Text` implies that it's not sent except in response to status deletion, but actually this doesn't seem to be the case; it also appears to be present in responses to creations and normal fetches and stuff. So I'm treating `ContentType` the same here.

* Update new tests to check content type on API statuses

* Check content type of API statuses in all tests where text is checked

* update other api tests with status content type field

* Add test ensuring text and content type are returned when deleting a status

* Convert processContentType to free function and remove unused parameter

* check for the correct value in the deletion test

* Be explicit about this test status having an empty content type

* Use omitempty consistently on API models

* clean up the final diff a bit

* one more swagger regen for the road

* Handle nil statuses in processContentType

* Don't pass processContentType the entire edit form, it doesn't need it

* Move processContentType to common.go and use for creation as well

* Remove unused parameters to ContentTypeToAPIContentType

internal/api/client/statuses/statussource_test.go

@@ -92,7 +92,8 @@ func (suite *StatusSourceTestSuite) TestGetSource() {
 	suite.Equal(`{
   "id": "01F8MHAMCHF6Y650WCRSCP4WMY",
   "text": "hello everyone!",
-  "spoiler_text": "introduction post"
+  "spoiler_text": "introduction post",
+  "content_type": "text/plain"
 }`, dst.String())
 }