Add images and styling similar to the email verification email to group invite email

Problem to solve

When inviting someone to a group via email, email invitation from gitlab.com looks very spammy.

Intended users

Anyone inviting new user via email

Proposal

Add images and styling similar to the email verification email so that Gitlab branding is apparent.

Permissions and Security

Should not affect Gitlab instance permissions or security.

Documentation

None

Testing

None

What does success look like, and how can we measure that?

Success would be invitees not having to contact the inviter every time after receiving the invitation to make sure it's not "some virus".

Perhaps CTR of invite links could be measured.

Links / references

None. Just personal experience. I have resorted to just telling people to join gitlab and tell me their username before I bother adding them. Adds a few cycles, but it's a good workaround.