Add ability to change the text on the 'Create Room' button

For example, an admin may want to change this to 'View Demo', 'Get Started', localize the call to action in another language.

There may be other areas that we want to allow people to change text beyond what is already surfaced, but this is the first one that's stood out to me so far that I wasn't able to do yet.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/HUBS-1048)

The issue does not name a file, test, or entry point. Start by locating the frontend implementation of the “Create Room” button and the existing surfaced text configuration, then check how localized strings are handled. Done means an administrator can configure this label, including localized alternatives, without changing unrelated button text.

Written by the indexing model from the issue text.