twitter: remove requirement of two internal file and support multiple searches and users

[strifel]

This PR removes the necessity to have to internal config files in twitterbot, by using different keys for searches and twitter_names in the same file. This does also fix #697 in the same way. See https://chat.zulip.org/#narrow/stream/127-integrations/topic/Twitter/near/1198859 for discussions.

As I do not have access to the Twitter API I could not test this PR.

I could see an possible error at

SINCE_ID_NAME = "since_id_" + opts.search_terms

as search_terms (or twitter_name) contains special characters like , or @