clearer and/or separate reviewer mail

[Shaoranlaos]

Issue description

Feature Request

Problem to be solved

The mail from the review-plugin that indicates an open review to a user (wether that is in the created-pr or in the modified-pr mails when a reviewer was added after creating the PR) doesn't realy show what actions the receiver of the mail has to take (if any).

Proposed solution

It would be more clear if the mails were more indicative what the user should do (if he/she has to do anything).
For this it would proberly be better to change the two mail-template for create-pr and modify-pr like follows:

• create-pr: Add a button into the reviewer section that links to the PR and has a label of something like "Review now".
• modify-pr: do not sent this mail type if only a reviewer was added to the PR (the current mode sents a "modified" mail with two identical text blocks which is confusing to the user), instead sent a separate mail that is similar to the create-pr mail but requests the user to review the PR

It colud also make sense to replace the create-pr with this separate review-request mail for all reviewer on the PR.