outgoing_webhook: Separate command and text fields.

[salandei]

This separates the slack legacy text field for slack-compatible outgoing webhook into command and text fields.
The command field contains the bot mention and the text field contains the content of the message after the bot mention. For better compatibility, the bot mention is transformed into Slack's slash command. e.g @mybot -> /mybot Test case for functionality is updated and api docs on the Slack-compatible format is updated.

Fixes: #19589

Screenshots and screen captures:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[alya]

@timabbott do you want to review this directly, or wait for @laurynmm to take a look?

[zulipbot]

Hello @zulip/server-integrations members, this pull request was labeled with the "area: integrations" label, so you may want to check it out!

[laurynmm]

There's an outstanding question about these changes impacting the python-zulip-api repository here: https://chat.zulip.org/#narrow/stream/127-integrations/topic/gitlab.20slash.20commands/near/1792657.