Implement resolving a topic

This issue describes a specific implementation for #11154. See also CZO discussion thread.

We should implement marking a topic as resolved by making it easy to prepend a “check mark button” emoji (✅) to the topic name.

Note that this feature makes it much more urgent to resolve #15290.

Three-dot topic menu in left sidebar: [for users with permissions to edit topic name only]

1. If topic name does not start with a ✅, show “Mark topic as resolved” below the “Move topic” menu option, using a checkmark as the icon. Selecting this option:

• Renames the topic by prepending a ✅.
• Sends a Notification Bot message to the new (resolved) topic that says: “This topic was marked as resolved by [user].”
• Does not send a message to the old (unresolved) topic, since we want it to disappear from the list.

1. If topic name starts with a ✅, show “Mark topic as unresolved” below the “Move topic” menu option, using a checkmark as the icon. Selecting this option:

• Renames the topic by removing all initial ✅.
• Sends a Notification Bot message to the new (unresolved) topic that says: “This topic was marked as unresolved by [user].”
• Does not send a message to the old (resolved) topic, since we want it to disappear from the list.

Compose box: When user starts composing a message to a topic whose name starts with ✅, show a warning banner in the style of the banner shown when @-mentioning a user who is not a member of the stream.

Text: You are composing a message to a resolved topic. For users with permissions, also show an “Unresolve topic” button on the right side of the banner.

[timabbott]

Yeah, I expect we’ll want to add something like that probably in the form of a “Who can resolve topics” setting, with one option being nobody, since I expect there will also be demand for limiting who has access to do it.

[timabbott]

@Signior-X would you be up for implementing the compose_error work, which I think is all that remains here? I figure you’re the best person to do it since you’ll want to serialize it with splitting compose.js in some reasonable way 😃.