Option: TeSS acknowledgments

[kennethrioja]

Summary of changes

• tess.example.yml: added about_tess_acknowledgments option, if true, displays displays ELIXIR TeSS acknowledgments in the bottom /about
• en.yml: added the relevant texts
• amendedtess.html.erb and created _about_tess_acknowledgments.html.erb to display it
• Added the relevant tests in about_controller_test.rb

Motivation and context

Good idea from https://taxila.nl/about#link_tess, doing it thanks to them. IMO it should be mandatory for any other TeSS instance

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.

[fbacall]

It might fit better if this was in the "Acknowledgements" section, what do you think?

[fbacall]

I think change this text to something a bit milder... like:

<site name> is based on the <a href='https://elixir-europe.org/' target='_blank'>ELIXIR</a> TeSS platform.