bots: Introduce System bots in UserProfile and port realm_internal_bots.

[adnrs96]

We introduce System bots as a new kind of bots in UserProfile. Following this we make realm_internal_bots use this new bot type while migrating any already created realm_internal_bot to new bot_type.

This is the first step towards moving from a world of cross_realm_bot to realm_internal_bots. In future we can make cross realm bots use this new type of bot type and migrate them to be realm based.

[adnrs96]

I have tried to cover all the spots where bots are exposed from to prevent SYSTEM_BOT profiles to be sent to frontend. This way we are trying to ensure that system bot types which aren't really meant for people to have access to don't get access to. Practically this just avoids confusing UI. Like reminders bot showing up in the bots list with a option to deactivate. Clicking the deactivate won't really deactivate the reminders feature though.

[zulipbot]

Heads up @adnrs96, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.