bridges: Check if the bot is subscribed to the relevant stream on startup.

[rht]

Currently, the check only exists on IRC bridge. We should do the same for the rest of the bridges. And while we are at it, we should have a reminder mechanism for a new bridge developer to ensure that the checking happens.

• bridge_between_zulips
• bridge_with_matrix

[rht]

cc: @orientor

[orientor]

@rht Added PR for the two bridges. To ensure that future developers use this I think we should add the check_subscription_or_die method to the zulip library. Also I think that the docs should be edited accordingly.