Web IDE button missing from project pages

Closes #36232 (closed), #36233 (closed)

Summary

Web IDE button now requires the project to have merge requests enabled.

Steps to reproduce

Turn merge requests off for a project. Observe the "Web IDE" button disappears from the project page. Turn merge requests back on for a project. The "Web IDE" button re-appears.

Example Project

https://gitlab.com/MJPA/empty-project - has merge requests disabled and no "Web IDE" button.

What is the current bug behavior?

No "Web IDE" button on the project page.

What is the expected correct behavior?

A "Web IDE" button on the project page.

Results of GitLab environment info

GitLab version 12.4.1

More info

New behaviour introduced by 8a008f80 as part of issue #29835 (closed)

I'm not sure if this is a bug or a change in requirements, but the Web IDE button is still there when looking at a particular file even with merge requests disabled. Example: https://gitlab.com/MJPA/empty-project/blob/master/README.md

Proposal

That means we would have to do the following things to get the Web IDE working for repositories with disabled merge requests:

1. Fix Error fetching merge requests from master popping up as soon as the user opens the Web IDE
2. Fix Error loading merge requests. Please try again popping up when the user opens the Branch / Merge request dropdown
3. Implement the disabled "Start new merge request" checkbox

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