feat: Redesign workspace join

[ArtyomSavchenko]

Suggest to join using current account if user is alredy logged in:

Rename "Sign Up" and "Join" buttons to avoid ambiguity:

[huly-github-staging]

Connected to Huly®: UBERF-15520

[aonnikov]

The Would you like ... %USERNAME% sentence is too long, can we redesign the page to be:

%USERNAME%      -- this can be displayed on every login page in the header
You are invited to join %WORKSPACENAME%

<Join %WORKSPACENAME%>
<Change account>

Also on Join / Signup and Join pages it would be nice to have workspace name.