Step 2 - Add alert and disable when cannot push code in IDE

• Step 1 - Clean up commit_sidebar/form_spec and commit constants
• Step 2 - Add alert and disable when cannot push code in IDE

What does this MR do?

It's possible for user's to visit the Web IDE without being able to push code. This can create a frustrating experience when they try to commit changes and discover an error on commit (see #213581 (closed)).

This MR addresses this problem by adding an alert box and disabling the commit buttons when the user cannot push code.

Heads up! It does not add the "Fork" button since this is a bit more involved. This will be handled in a separate step.

Screenshots (strongly suggested)

On load:

User can push changes	User cannot push changes
20210118_ide_user_can_push	20210118_ide_user_cannot_push

Editor:

User can push changes	User cannot push changes

Begin commit button:

User can push changes	User cannot push changes

Commit tab:

User can push changes	User cannot push changes

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

References

• Related to #213581 (closed)