* Improve text & add icons

* Previously a star wildcard was applied in the css, eliminating margins on all header elements when it shouldn't (e.g. Log in text and icon spacing). We use specific class margins in the html template instead.

Add SLS document link (Only in German for now)

* this caused issues on the draft preview page
* when a draft didn't have a published record with the same ID, the new
  filters couldn't find any results and an exception would be raised
* this exception was caught by the registered error handlers and masked
  as 404

* via custom jinja filters for resolving the record and its first owner

* From now on, we use external links instead of local pdfs

* Add 2 extra logos with their respective links
* Apply styling
* Add static images: Use webp format and png format to have a reference for the original images
* Add an extra route for terms of use, instead of the external pdf link
* Update pdf links: use external links, instead of serving them locally

* Use 'TU Wien Research Data' as the default config value. The previous one isn't valid anymore.

* avoid a flash of invisible text (FOIT) by using temporarily using
  system fonts until the fonts have been loaded

* the current versions of invenio-accounts and invenio-oauthclient (both
  v2.0.1) use the same 'blueprint.record_once' mechanism to modify the
  config values, leading to race conditions with our hack
* the config override has been moved to invenio-config-tuw

* remove old documents
* hide the privacy statement from the footer for now

* take advantage of `blueprint.record_once()` for setup logic, which is
  called during registration of the entrypoint blueprints (which happens
  after the invenio extensions have been loaded)
* this pattern is already used in some core modules
  (e.g. invenio_rdm_records.views v1.0.0)

Closes #24

* although still technically true, it makes the system sound like an
  early alpha/beta version, which it is not anymore

* instead, we override the app's config in a way that evaluates the
  SITE_{API,UI}_URL values and casts them to strings
* because we have configured these values as LocalProxy objects with
  some logic for dynamically calculating the URLs based on the HTTP
  request context instead of plain strings, we sometimes ran into typing
  issues
* the above mentioned logic helps us with having multiple valid domain
  names (and generating valid links based on which is being visited)

* they're now passing custom_fields

* create a config item THEME_TUW_FRONTPAGE_WARNING for the content
* add a block in the frontpage.html
* apply styling

* to reduce their file size, for transmission speed
* also fix the left border on the policies hero

* Searchbar now includes total record count in its placeholder
* Style enhancements to make placeholder text more noticeable

Closes #32