Closes #31073

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/30149

Use more semantic `<button>` element and prevent the form from
submitting.

This will only hapen in certain situations ie. when the star count is a
lot. We were previously fixing it by hard coding a max-width.

This changes it to use flexbox which allows the content to decide when
it should be truncated. The rows don't always need truncating, so we
shouldn't hard code a width.

Closes #29018

Now you can create a new file and select a target_branch != source_branch.
If the file that you want to create already exists on the target branch an error message is shown
A glDropdown is used to select and create a new branch instead of a text field.

Also created changelog and removed redundant code

Added a tooltip to the buttons that did not have any

Closes #28572

This reverts merge request !9338