Content update for 'Info' tab headline

Rename headline from 'Summary' to 'Info'
Create template for 'Info' tab

- Provide functions to create 'section'-, 'aside'- and 'footer'-
  elements in tab.js
- Create 'section'-, 'aside'- and 'footer'-element for Info-Tab
  in info.js
- Info-Tab now provides information about episode-details, show-details
  (titles, subtitles, publish-date, urls, duration, show image),
  a subscribe button and copyright/licence details
Add timestamp parameter to example.html
Tab-template: Add icon to each tab headline as a pseudo-element
Upload images for social links in info tab
Set flexible layout to active tabs per default

- Include flexbox-layout- mixin to class .tab.active
- Add icon as pseudo-element before the main tab headline
- Fix margins of tab-header and tab-headline
- Add classes .main and .section elements to flex items in tab container
- Clean up code
Create UI component: Flat button with rounded corners

- Create new mixin for flat button
- Create new mixin for all 4 rounded corners
- Set font-size variable for button labels
- Clean up code: Delete vendor prefixes in scss
Adjust font-weight in headlines and em-elements
Delete unused imports in podlove-web-player.scss
Add mixins "row", "container", "column"
Refactor flexbox mixin

- Include flexbox mixin wherever needed
Refactor module info.js

- Indent code in a more readable way
- Add date object to render a publication date
- Add div in info-tab-footer with social links
Styling for info tab

- Apply flexbox layout to elements in info tab
- Style buton and footer
- Fix cross-browser issues
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