Require --force to claim an issue without "help wanted"

[rishig]

We've started getting a lot of contributors claiming issues that don't have the help wanted label.

It would be great to automatically respond to @zulipbot claims on those messages with:

Welcome! 
You have attempted to claim an issue without a `help wanted` label. If you are new to Zulip, we recommend picking an issue with the [help wanted](link) label, or [good first issue](link) label. We also recommend reading our [guide for new contributors](zulip.readthedocs.io/en/latest/overview/contributing.html) before getting started.

To claim this issue anyway, you can use `@ zulipbot claim --force`.

[vibhorgupta-gh]

@zulipbot claim

[zulipbot]

Welcome to Zulip, @VibhorCodecianGupta! We just sent you an invite to collaborate on this repository at https://github.com/zulip/zulipbot/invitations. Please accept this invite in order to claim this issue and begin a fun, rewarding experience contributing to Zulip!

Here's some tips to get you off to a good start:

• Join me on the Zulip developers' server, to get help, chat about this issue, and meet the other developers.
• Sign the Dropbox Contributor License Agreement, so that Zulip can use your code.
• Unwatch this repository, so that you don't get 100 emails a day.

As you work on this issue, you'll also want to refer to the Zulip code contribution guide, as well as the rest of the developer documentation on that site.

See you on the other side (that is, the pull request side)!

[zulipbot]

ERROR: You have not claimed this issue to work on yet.