templates: Add list of steps to new contributor message.

The goal here is to encourage folks not to claim an issue until they
are ready.

We will need to make sure that they don't get assigned the issue if someone
else claims it in the meantime.

We're counting on the contributor not to accept the GitHub invite until
ready, which is not ideal because (a) they might just accept it, and
(b) there's a risk that the invite will expire. So we'd probably be better
off splitting this into two interactions (joining repo; claiming issue).

This also shortens the message if the initial invite hasn't been accepted.

config/templates/contributorAddition.md

@@ -1,10 +1,9 @@
-Welcome to Zulip, @{commenter}! We just sent you an invite to collaborate on this repository at https://github.com/{repoOwner}/{repoName}/invitations. Please accept this invite in order to claim this issue and begin a fun, rewarding experience contributing to Zulip!
+Welcome to Zulip, @{commenter}! To get started, please take the following three steps **in order**:
 
-Here's some tips to get you off to a good start:
+1. Complete all the steps in the [Your first codebase contribution](https://zulip.readthedocs.io/en/latest/overview/contributing.html#your-first-codebase-contribution) section of the [Zulip contributor guide](https://zulip.readthedocs.io/en/latest/contributing/index.html) up to "Claiming an issue".
+2. Post a comment here describing your plan for how to approach this issue. Before you claim an issue, you should be confident that you will be able to tackle it effectively.
+3. Accept the invite to collaborate on this repository at https://github.com/{repoOwner}/{repoName}/invitations. To avoid filling your inbox, we recommend [unwatching this repository](https://help.github.com/articles/unwatching-repositories/).
 
-- Join me on the [Zulip developers' server](https://chat.zulip.org), to get help, chat about this issue, and meet the other developers.
-- [Unwatch this repository](https://help.github.com/articles/unwatching-repositories/), so that you don't get 100 emails a day.
+The issue will be assigned to you when all three steps have been completed.
 
-As you work on this issue, you'll also want to refer to the [Zulip code contribution guide](https://zulip.readthedocs.io/en/latest/contributing/index.html), as well as the rest of the developer documentation on that site.
-
-See you on the other side (that is, the pull request side)!
+If someone else claims this issue while you're working through the checklist, no worries! You can contribute by providing feedback on their pull request. If you’ve made good progress in understanding part of the codebase, you can also find another “help wanted” issue in the same area to work on.

config/templates/inviteError.md

@@ -1,3 +1 @@
-Hello @{commenter}, it looks like we've already sent you a collaboration invite at https://github.com/{repoOwner}/{repoName}/invitations, but you haven't accepted it yet!
-
-Please accept the invite and try to claim this issue again afterwards. We look forward to your contributions!
+@{commenter} It looks like we've already sent you a collaboration invite at https://github.com/{repoOwner}/{repoName}/invitations, but you haven't accepted it yet. Please accept the invite, and then try to claim this issue again. Thanks!