Remove !important property from message.css and add description in inmail.js

Problem/Motivation

In #2822297: Allow select Plain text / HTML message body display we've committed also a css role with !important.

It is to be avoided whenever possible as it limits code from easily extending / overriding it and thus a last resort of custom code.

And since this could be a cleanup followup, let's improve the js by adding the missing "second docblock" explaining what the tabs part is doing.

Proposed resolution

- Remove the !important property from the message.css

+++ b/css/message.css
@@ -52,6 +52,20 @@
+.inmail-message__body .ui-tabs-nav {
+  padding: .2em .2em 0 !important;
+}

- add doc for inmail.js tab part

Remaining tasks

User interface changes

API changes

Data model changes