Wizard for configuration?

[renatolond]

For #36 we will probably need to ask for the minimal permissions when linking the accounts and then asking for more as the user chooses to post in one or another service.
Which leads to the idea that we should have a "Wizard" for configuring the crossposter.

1. Link accounts
2. Would you like to post from Mastodon to Twitter?
   2.1) Increase Twitter permission to allow posting
   2.2) Show options to post from Mastodon
3. Would you like to post from Twitter to Mastodon?
   3.1) Increase Mastodon permissions to allow posting
   3.2) Show options for Twitter
4. Remind user that the permissions can be changed at any time