Supply redirect_uris as a string for greater compatibility

Tokodon only uses one redirect URI at a time, namely either urn:ietf:wg:oauth:2.0:oob or tokodon://oauth. The Mastodon client API documentation states that redirect_uris can be a string or an array of strings: https://docs.joinmastodon.org/methods/apps/#form-data-parameters

However, other software implementing the Mastodon client API do not accept arrays of strings for this parameter:

• GoToSocial takes a URI or a newline-separated list of URIs, but not an array
• Sharkey only allows strings, not arrays

I have confirmed this change enables a successful login to a GoToSocial instance. I am not currently in a position to test this change successfully enables login on Sharkey. I have confirmed that this change does not break Mastodon compatibility. I do not know of any Mastodon client API implementation that would lose Tokodon compatibility from this change.