Credits should be customisable in config

In User Settings > Help and About > Credits, the credits (for default cover photo and emojis), seems to be hard coded in the [HelpUserSettingsTab.js](https://github.com/matrix-org/matrix-react-sdk/blob/develop/src/components/views/settings/tabs/user/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 `

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.