Minor missed issues with 747

[taoeffect]

Problem

I missed a few minor issues with #747 in my review, describing them all here:

Issue 1

{tag_}{var}{_tag} pattern

Issue 2

When no explanation is provided for why a person is being added to the group, empty quotes are shown:

Issue 3

When visiting the link shown above in a private window, with no user signed up, instead of the welcome page we are shown a broken link:

Issue 4

Avatar error in console after group creation:

Solution

1. Move {strong_} and {_strong} out of the string-to-be-translated and into the {username} variable as surrounding HTML
2. Hide the explanation completely when it's empty
3. Make sure the welcome screen is shown to new users who haven't yet signed up for links generated through proposals, and add a test to catch this in the future
4. Change or remove the default avatar URL

[taoeffect]

Note: edited issue to add issue 4 related to the avatar