Rewrite ChangeSuggestion logic

[BenjaminBrandtner]

Any visitor to the website may submit suggestions for adding new TranslationChannels or changing the tier or goodEditor status of an existing one. They will then be considered by members in Nova.
The logic I've already written for this is incomplete and different from how I picture the proccess now.

The process should be like this:

A page on the frontend titled "Submit a channel"
At first, only a url field is shown. Filling it out makes an Ajax Request. (POST at /api/translation-channels/search)
Validate that it's a youtube channel url. If not (Response is 422), display validation error.
If valid (Response is 200 or 404), display the rest of the form and a submit button.

Also, display the following next to the full form:
TranslationChannel already exists: ChannelBubble
ChangeSuggestion with that channel_id already exists (POST at /api/change-suggestions/search): "This channel has already been submitted and is awaiting approval."
Else: "This is a new submission."

table change_suggestions
channel_id (string), tier, good_editor

At no point in this process should the YoutubeAPI be called.

[BenjaminBrandtner]

The api should be complete, only the frontend is missing now.