Update contributing docs and add commit squash instructions
Bug 856922 - Enable Contribute page for Arabic

* Add email address to LOCALE_CONTACTS
* Add locale to LOCALES_WITH_MOZ15
* Add custom banner (css rule+png file)
* CSS rtl:
** fix buttons in nav (font too small, margins), areas of interest (order of elements)
** fix form fields order
** fix checkbox in subscription section
Bug 925278: add gl to channel page redirect
Bug 892470: enable plugincheck for gl
Merge pull request #1608 from flodolo/channel_gl

Bug 925278: add gl to channel page redirect
Merge pull request #1609 from flodolo/gl_plugincheck

Bug 892470: enable plugincheck for gl
[fix bug 952686] Take down security-group page
Redirect /mobile/credits to /credits/
Bug 961010
Merge pull request #1610 from sgarrity/bug-961010-mobile-credits-redirect

Redirect /mobile/credits to /credits/ for Bug 961010
Merge pull request #1600 from flodolo/bug856922_getinvolved_ar

Bug 856922 - Enable Contribute page for Arabic, improve CSS for RTL
Bug 906943, Fix GA non-interaction event parameter on Translation Bar
Fix Bug 890378 - Title wrapping on CJKT Manifesto pages
Bug 892470 - Enable /new for gl
Bug 892470: enable plugincheck for km
Merge pull request #1615 from flodolo/new_gl

Bug 892470 - Enable /new for gl
Bug 925278: enable channel page for et
Merge pull request #1616 from flodolo/plugincheck_km

Bug 892470: enable plugincheck for km
Merge pull request #1617 from flodolo/channel_et

Bug 925278: enable channel page for et
Merge pull request #1607 from alexgibson/take-down-security-group

[fix bug 952686] Take down security-group page
Merge pull request #1602 from alexgibson/add-rebase-contributor-instructions

Update contributing docs and add commit squash instructions [no bug]