Create MR view doesn't let me set the source branch to a branch from my fork
## Summary
From the Create MR view, I cannot choose my own fork of the project for the branch I'd like to merge
## Steps to reproduce
1. Fork a project.
1. Make a new branch off master in your fork.
1. Commit a change to that branch.
1. Go to the original project's Merge Requests page.
1. Click "Create New Merge Request".
1. Note that you can't create a new MR from the branch on your fork.
## Expected behavior
I should be able to choose from my fork(s) of the project in the "source project" selector.
## Relevant logs and/or screenshots

## Possible fixes
Add forks the user is a member of to the possible "source projects" list.
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