Do not list all domains for custom verifications

Currently many Mastodon servers are shown when you want to trigger a custom proof. This is, by no means, works if you want all hundreds of Mastodon servers etc. in there.

Also, if other services get added, you have no way to manually find that one in the list of all that Mastodon servers.

Solutions:
* just allow any domain for input
* only use input as a search and show results after 3 character or so. Do not show a huge scrollable list, in which nobody finds anything anyway.
* IMHO, preferable: First let the user select a service (e.g. Mastodon) and only then let them enter domain/search.

No file or test is named; start by locating the custom verification flow and its Mastodon domain selector in the client or website. Compare the proposed service-first and search-based options with the existing UI, then verify that large domain lists are no longer shown and that users can find a specific service and domain.

Written by the indexing model from the issue text.