Refactor Tab.createHeader
generalise and move CSS rules for tabs

Only rules that apply solely to the info tab are left in _info.scss.
tab module methods return appended elements

`Tab.createHeader`, `Tab.createFooter`, `Tab.createSection` and
`Tab.createAside`,
refactor info module

Render social links, license information and subscribe button only when
possible.

Define social link types with class name, title and image.

Add helper functions for each subtask.

* createEpisodeInfo
* createPosterImage
* createSubscribeButton
* createShowInfo
* createSocialLink
* createSocialInfo
* createSocialAndLicenseInfo
refactor chapter module

Put all private methods at the top.

Append chapter table to main content to fix display.
fix display of download tabs

Call `Tab.createSection` to fix the display of the downloads tab.
fix display of share module