Whitespace permitted in branch patterns

Issue Summary

When creating branch patterns in default reviewers, whitespace characters are permitted.

Steps to Reproduce

1. Create a new default reviewer rule
2. Create a branch pattern for either the target or source branch that contains a whitespace character

Expected Results

A validation error should indicate whitespace is not permitted in branch patterns (as they are forbidden in git branches and will never match anything)

Actual Results

The rule is created successfully.

If whitespace precedes the rest of the branch pattern (e.g. " some-branch"), the whitespace is trimmed from the UI, which makes the pattern appear valid (see screenshot). As the branch pattern does not match any branches, the rule will not apply default reviewers, even if the branch appears to match the pattern in the UI.

Workaround

The whitespace-containing rule must be deleted and recreated.