Add saved snippets

[alya]

In zulip/zulip#31227, we are implementing saved replies in the web app.

It would be useful to be able to insert a saved reply in the mobile app as well.

For the initial implementation, we will leave out filtering snippets.

API discussion

CZO discussion

Figma design

[gnprice]

Our current plan is that we'll skip this feature for the legacy app, but will have it in the new app at launch time. That way in the blog post for the upcoming Zulip Server 10 release, we can say that mobile support for it is coming soon as part of the new app.

Ideally this means we'd have the feature in the new app, or at least have started implementing it here, by the time of the Server 10 release in order to be sure there are no surprises before launch in how much work it takes.

[PIG208]

(from the changelog, FL 297):

• GET /events, POST /register: An event with type: "saved_snippet" is sent to the current user when a saved snippet is created or deleted.
• GET /saved_snippets: Added a new endpoint for fetching saved snippets of the user.
• POST /saved_snippets: Added a new endpoint for creating a new saved snippet.
• DELETE /saved_snippets/{saved_snippet_id}: Added a new endpoint for deleting saved snippets.