Fix initial letter avatar vertical offset in Firefox
In at least Firefox, the case of relative positioned inline elements (such as
mx_BaseAvatar) with absolute positioned children (such as mx_BaseAvatar_initial)
is a dark corner full of spider webs. It will give different results during full
reflow of the page vs. incremental reflow of small portions. While that's surely
a browser bug, we can avoid it by using `inline-block` instead of the default
`inline`.
Fixes https://github.com/vector-im/riot-web/issues/5594
Might help with https://github.com/vector-im/riot-web/issues/9088
See also https://bugzilla.mozilla.org/show_bug.cgi?id=1535053 and
https://bugzilla.mozilla.org/show_bug.cgi?id=255139 for more details on browser
behavior in this case.
990d6dec15