Use diaspora_id validation from the federation gem

DRY

fetch mentioned people if they don't exist locally yet

Fixes #6979

Allow hovercards of remote people when not logged in

Improve /tag page performance.

In #6603 hovercards were activated for logged out users, but they only
worked for local people. Hovercards on remote-people are important to
get the diaspora-id to search the person on the local pod, if you don't
know who it is.

Cleanup old rake tasks

* accounts:run_deletions: was added with 0.4.0.0 two years ago for one-time usage.
* db:rebuild: db:reset does basically the same
* db:integration:preprare: the integration environments are not used.
* db:reset: there is a default db:reset, no need to write our own.
* db:drop_integration: the integration environments are not used.
* db:fix_diaspora_handle: really old migration from 2010
* db:move_private_key: also old migration from 2010
* maintenance:clear_carrierwave_temp_uploads: tmp/uploads doesn't exist anymore. And we have CleanCachedFiles as cronjob.
* maintenance:install_logrotate_config: diaspora has built-in logrotate support now, and people who want to use logrotate instead can write their own configs with the values they want.
* migrations:copy_hidden_share_visibilities_to_users: old migration from 2012
* migrations:invitations: legacy invitations were removed with #6976
* migrations:absolutify_image_references: old migration from 2010
* migrations:rewire_uppercase_hashtags: old migration from 2012
* migrations:remove_uppercase_hashtags: old migration from 2012

Don't create notifications from ignored people

Also move "shareable hidden"-logic to AlsoCommented, because it is the
only one that needs it. And write some specs for mentioned and started
sharing notifications.

Fixes #6294

Display markdown images inline

Update gems