Wiki search form improvement

In Moodle 3.9 the style of wiki search form differs from the style of surrounding user interface elements. While other elements benefit from using bootstrap classes, wiki search form does not.

We made some improvements to the code. The patch file attached.

Improvements made:

• According to suggestion to use mustache templates given here https://docs.moodle.org/dev/Templates we implemented mustache template for wiki search form.
• Wiki search form has its renderable class now.
• We used bootstrap classes in the template. Like in search form in forum module. The search form suits the user interface better.