Update RTD theme and add jsPsych tab

[becky-gilbert]

This PR updates the Sphinx RTD theme that we're using and adds some customization for the EFP and jsPsych "tabs" above the navigation menu, which link to those separate docs pages. Once we're happy with how this customization looks, we can add to the EFP and jsPsych docs separately so that those sites look the same (#435).

Here are the changes:

• Removes the existing RTD theme. This was a forked copy maintained here: https://github.com/kimberscott/sphinx-rtd-theme-frameplayer
• Adds a customized layout template for the standard sphinx-rtd-theme. This extends the default layout but overrides the sidebartitle block with a version that combines the default contents with the custom HTML for tabs (copied from the old forked version).
• Adds a project name above the search bar: "CHS/Lookit"
• Changes the "Lookit" tab label to "CHS"
• Changes the "Experiment Runner" tab label to "Lookit EFP"
• Adds a "jsPsych" docs tab
• Removes the margin-right on the tab elements so that they fit horizontally in the navigation menu
• Updates the package requirements

With changes in this PR

Current version, for reference