Add Waffle Flag for Submissions

[chrstinalin]

Fixes: mozilla/addons#1830

Description

Adds waffle flag enable-submissions to disable add-on related submissions (both new add-ons and versions) when toggled to false for a particular group on both DevHub and the API. Includes the endpoints:

1. POST api/v5/addons/addon/
2. PUT (CREATE) api/v5/addons/addon/<guid>/
3. POST api/v5/addons/addon/<guid>/versions/
4. POST api/v5/addons/addon/<guid>/previews/
5. PUT api/v4/addons/
6. POST api/v4/addons/<guid>/versions/<version>
7. PUT api/v3/addons/
8. POST api/v3/addons/<guid>/versions/<version>

The waffle flag's 'note' can be used to add additional information. The API response:

{
    "error": "Submissions are not currently available.",
    "reason": "This is the note reason."
}

And on DevHub:

Testing

1. Deactivate the enable-submissions waffle flag (everyone should be = True by default).
2. On DevHub, all URLs pointing to a) uploading a new add-on, or b) uploading a new version are blocked.
3. Any API requests to endpoints that do the same return the error.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).

[diox]

Can you refactor the conditions in the views into a decorator or two (one for APIs, one for "regular" views) to reduce duplicated code ?

[diox]

I think we should be a little more subtle than that (sorry it was so poorly specified in the issue):

• We don't need to completely block all steps of the submission process. Let's only block the part where the add-on is being submitted, that's what matters - for the rest, it's too late anyway.
• On the page where the developer is selecting the file, let's allow access to the page if it's a GET, but disable the inputs/buttons and show the error message there. Or even replace the entire with the message, whatever is easier. That way developers should never be surprised (and in theory shouldn't hit the 403 submissions_disabled.html page, unless they tried to be sneaky or submitted right when the flag was modified)

[diox]

Wrong button before, didn't mean to approve, definitely need changes.

[diox]

We can ship this, but create an issue to address these points in a follow-up:

• The Theme Submission page (/en-US/developers/addon/submit/theme/upload-listed) has the Create a theme button still enabled when the flag is off
• The Theme Wizard page (that the aforementioned button leads to - /en-US/developers/addon/submit/wizard-listed) also has its submit button still enabled
• Some warning should be displayed somewhere on the upload & theme wizard pages and not just on the title of the disabled buttons, to make it more obvious. With the full note for consistency.