When installing themes using the "Install this theme component" button
on meta.discourse.org, we pass the repo name and URL via query params.
However, these stick. So if a user cancels the installation, on the
next navigation to the same route, they'll see the modal again.
This PR clears the query params of the controller when dismissing the
modal.
When installing private themes and theme components, the public key does
not show until the administrator types a valid Git repo URL. The regular
expression that checked the URL was too strict and it required the URL
to end with ".git".
