Accessibility improvements InclusiveOCW

[aoelen]

Made some changes in order to improve accessibility. This includes recommendations from the DZB document from 11 Jan 2019.

This includes changes to Semantic UI (see the semantic-ui-customized repo).

[aoelen]

Some more information about what changes are made in this PR. As mentioned in the PR, the tasks performed were from the DZB report.

Done

1. Home & Search results & Presentation landing: change alt text of featured deck to a more understandable text (from "featured image" to "Example of presentation: name")
2. Home: no tab focus possible anymore on the featured image (improves navigation, requires less tabs to navigate through the page)
3. Presentation overview: updated descriptive text of Creative Commons Logo, removed unneeded alt options
4. Home: updated heading of featured deck (a more logical order, from h2 to h4)
5. Home: make heading more uniform (both h2 and similar appearance)
6. About, Accessibility Statement, Imprint (and all other static pages): changed headings from h3 to h2 where needed and added headers where needed
7. About, Accessibility Statement, Imprint (and all other static pages): using semantic aside element for the some elements
8. Presentation overview: h2 menu changed to regular text
9. Signup: changed heading from h2 to h1 where needed
10. Imprint: changed b-tag to em-tag
11. Presentation view: added aria-current="page" to the current slide number
12. Search results: changed text colors to have more contrast: placeholder text of inputs, user account text, not available text
13. Presentation overview: changed text colors for contrast in activity feed
14. Search results: removed double tax-index around "Advanced Options" button.
15. Presentation view: removed role="application" and aria-live="polite" from the slides
16. Home: meta title has been changed to a more useful text (this make clear for the user where he/she is at the moment)
17. Presentation overview: clearer focus on the language selector
18. Sign in dialog: better focus color on Google sign button
19. Registration: errors are moved to the top of the dialog
20. Sign in dialog: clearer input fields, labels are hidden now, and placeholders are used to describe the fields
21. Presentation landing, presentation overview, sign up: WCAG related errors fixed
22. Impress: added "mailto:" to links of email addresses
23. Presentation overview: currently selected language is describe by the aria-label attribute

Todo

1. Hard-coded text needs to be converted to translatable text
2. Text needs to be translated (73% of the text is translated to German)
3. Presentation overview and presentation landing page have the same title, needs to be changed
4. Presentation view: keep focus inside the slide navigation menu when opened
5. Forms error: connect errors to inputs fields with IDs
6. Aria-label vs. hidden text for screen readers only (https://accessible360.com/accessible360-blog/use-aria-label-screen-reader-text)
7. Search page: more accessible tag selector (e.g. for owners and tags on search page)

[YaserJaradeh]

The changes doesn't impact the system's performance negatively, and they seem to be working. Moreover, no functionality has been lost.