Update New Forem Member invitation email text and html

[ellativity]

Requesting the following changes to the new member invitation email that is sent when an Admin uses /admin/invitations/new to invite members.

Text lives here
HTML lives here

-We'd like to update the email so it's more personalized to the recipient (we should have a stored field in the Forem for their name as submitted by the Admin when creating the new invitation, as in here).
-Similarly, we would like to pull the Community Name of the Forem (as in here)

(I've tried to find where these things live, but I may have gotten the wrong code, so please don't rely on me for facts!)

Hello <name> (this is one of the 2 required fields for an invite so I don’t understand why we’re not using it)

You’ve been invited to join the <Forem Name> community at <forem URL>. You can accept the invitation by clicking this link:

Accept invitation

If you don’t want to accept the invitation, please ignore this email. Your account won’t be created until you access the link above and set your password.

[github-actions]

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