MAPG-69 make emails HTML explicitly

mail/signup.html

@@ -0,0 +1,13 @@
+Hi,
+<br><br>
+You recently signed up on MapGuesser with this email address ({{EMAIL}}). To activate your account, please click on the following link:<br>
+<a href="{{ACTIVATE_LINK}}" title="Account activation">{{ACTIVATE_LINK}}</a>
+<br><br>
+If you did not sign up on MapGuesser or changed your mind, no further action is required, your email address will be deleted soon.<br>
+However if you want to immediately delete it, please click on the following link:<br>
+<a href="{{CANCEL_LINK}}" title="Sign up cancellation">{{CANCEL_LINK}}</a>
+<br><br>
+Have fun on MapGuesser!
+<br><br>
+Regards,<br>
+MapGuesser