Improve localizability of 'Create merge request' button on 'New merge request' form

What does this MR do and why?

For #543136 (closed)

This MR stops splitting UI text between localizable and non-localizable strings, which fixes a number of issues with localization.

"Issuables"

While the original localization and text here assumes any type of issuable (which might be merge request, issue, incident, or task), in my testing this form is not used by the create forms for anything other than merge request. If that's not correct, we likely have a test coverage gap (since the tests are passing).

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