Fix copyright

TBD: think about UX and what to do when the copyright is set to empty, how to edit it afterwards.

We need to fix our copyright in footer. Right now it's copyright by jtalks.org, which should be changed to: "<foundation year> - <current year> Copyright by <domain>".

Under that there should be Powered by JCommune 2.3.1777.82b4691 by jtalks.org with link to jtalks.org.

Acceptance Criteria:

• Given I'm in Admin Mode, I can click on copyright at the bottom of the page and will be able to edit it.
• Given I've opened Edit Copyright, I see a single text area where I can specify new copyright text
• Given I see Edit Copyright Dialog, I have hints that I can use placeholder {current_year}

  which will be replaced by actual value on the fly

• Given I've saved new value, it's shown on the page at the bottom
• Given I've used placeholder for current year, it's replaced by the actual value of current year