Bug 1014219 - [Messages] Consider using real <button>s instead of <a> links for the header icons

The settings button on header, is changed from <a>  to button element  and button id has been changed according to the convention.
Total changes are done on 2 files.
It's better to keep ids in the following format: ```{view}-{function}-{type_of_control}``` eg. ```threads-options-button```. With this you won't need to change the way how element is retrieved below.