Add a "new thread" button on the plugin support forums

[TJNowell]

Migrating from ​#wp24517:

Having left a review on a plugin I wanted to report a bug, only to find no link to create a new thread.

After a while, I realised i could scroll down past what I initially thought was the bottom of the page, to find a submission form. As you can see from attached screenshot, it wasn't clear.

A new thread button towards the top would fix this by providing a point of action. It need only link to an ID further down the page. I've also attached a screenshot of a quick mockup I did in browser, an anchor with class button, floated to the right with appropriate margins did it, though a container around it for more buttons would be preferable

See also the screenshots in ​#wp24517.

We'd probably add it as a button-looking link that goes to an anchor where the form is on the page. That would be enough. Of course, there may be reasons *not* to do this, namely: it keeps people from searching/reviewing the forums first. Worth discussion.

[SergeyBiryukov]

Related: #1977

[coffee2code]

In 4721:

Support Forums: For plugin/theme support forum sidebars, add link to "Create Topic", "Add Review", or "Edit Review" based on context.

Adds an above-the-fold link that leads to the down-page form (which is otherwise only discoverable by scrolling).

Fixes #169.
See #1977.