bot: add archive bot

[da5nsy]

Introduces a new bot which makes an archive of a web link using archive.org.
I made this following a conversation in the integrations channel here where I said:

Is there a bot (or other mechanism) which could watch a channel or topic and automatically add any posted links to webarchive.org? (and ideally reply with the webarchive link?)

Context for the curious: I'm house hunting with my housing co-op, and we like to share links to zoopla, and it would be useful to look back at them for reference, but they get taken offline as soon as they're sold (which, in London, is often quite quickly...)

I haven't (yet) written tests. I will either need to learn how or get some assistance in doing so.

Making this PR, but leaving it as a draft for now, to get some feedback. This is my first PR on this project.

How did you test this PR?

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]

Thanks! What kind of feedback are you looking for? You can post specific questions in the #development help channel in the Zulip development community.

Is there an issue this PR is solving?

[da5nsy]

Wow thanks so much for the speedy reply @alya! Still working out how things work around here.
I've just added a link to the top comment adding a bit more context and a link to a previous discussion.