Render all page headers on backend, remove react-helmet component

[glenbraun]

We recently added support for rendering meta headers on the backend, and should do that for all pages

Checklist

• Add page handlers in context_preload.py

• Remove Header component on frontend

• Remove React Helmet dependency

Trello Card

[PeterBreen]

Here's some code examples of how to do this, as several pages already have backend meta tag rendering:

For the About page, the information is defined here:
https://github.com/DemocracyLab/CivicTechExchange/blob/master/civictechprojects/helpers/context_preload.py#L87-L91

and then we use FrontEndSection to make sure that information goes where it belongs:
https://github.com/DemocracyLab/CivicTechExchange/blob/master/civictechprojects/helpers/context_preload.py#L183

And the about page has the correct meta information:

The component we want to remove, Headers.jsx (which uses React Helmet) is here: https://github.com/DemocracyLab/CivicTechExchange/blob/master/common/components/common/Headers.jsx and only used in two places. Unfortunately, they're the two most complicated to replace - they need to respond to whatever project or group data is loaded.

[marlonkeating]

Fixed by #1018