Backport #29069
The old code `GetTemplatesFromDefaultBranch(...) ([]*api.IssueTemplate,
map[string]error)` doesn't really follow Golang's habits, then the
second returned value might be misused. For example, the API function
`GetIssueTemplates` incorrectly checked the second returned value and
always responds 500 error.
This PR refactors GetTemplatesFromDefaultBranch to
ParseTemplatesFromDefaultBranch and clarifies its behavior, and fixes
the API endpoint bug, and adds some tests.
And by the way, add proper prefix `X-` for the header generated in
`checkDeprecatedAuthMethods`, because non-standard HTTP headers should
have `X-` prefix, and it is also consistent with the new code in
`GetIssueTemplates`
