Credits should be customisable in config

[jamescl]

In User Settings > Help and About > Credits, the credits (for default cover photo and emojis), seems to be hard coded in the HelpUserSettingsTab.js file.

We should have a way to customise the credits to reflect changes made to a self installed Element.

In particular, if we're using welcomeBackgroundUrl in config.json, the default cover photo credit isn't relevant and we're not crediting the right people for their work.

There should be a way to configure this either through a file that contains the credits in html format, or via config.json e.g.

"credits": [
      { "order": 1, 
        "html": "*some html*"
      },
      {"order": 2,
       "html:": "*some other html* "
]

Where the values in html are in within the list item tags e.g <li>*some html*</li>

Also, this would mean that we could credit matrix.org for the spec and the element/new Vector team for being behind the application if the app admin wanted.