Fix IDE openMergeRequest to route to first file

What does this MR do?

This MR fixes #273067 (closed), where opening the Web IDE from an MR caused the opened file not to be opened in the file tree. While fixing this, this MR takes an approach that also fixes a few other quirks and sets us up for simplifying some very brittle actions in a follow-up.

Screenshots (strongly suggested)

Before (on master)	After (this MR)
20210212_ide_open_mr_before	20210212_ide_open_mr_after

• Notice in the before, I have to manually open a file to trigger it opening. Then it magically opens all the files it should have.
• Notice in the before, there's a delay when I switch tabs. This is because the call to getFileData doesn't actually cache the result we need (we also need to call getRawFileData (see relevant comment)).

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

References

Related to #273067 (closed)