Show a better message on the login screen after an initial sync failure.

fixes #86
fixes #89

Convert list of receipts to map<userId, timestamp>

fixes #131

When a custom filter is inserted into nheko.conf or there was no filter
defined yet the default filter gets automatically uploaded.
After a successful upload the server-side generated filter-id is used.

This is done async as it is just an enhancement and it is not required
to upload the filter before the first request.

[ci skip]

fixes #199

[ci skip]

I had a look at sync.cpp and checked which parts of the sync response
are currently handled and which not. As I think it is unnecessary to let the 
unhandled data be transmitted without being handled I added these filters.

In the same term I increased the timeout server-side to 30s as Riot
defaults to this value as well. Especially now when a lots of presence-updates 
are not send anymore this value is more relevant.

It is now also possible to use a filter that is defined in`client/sync_filter`.
Advanced users might want to set an own filter here.

[ci skip]

fixes #132