Remove modified concepts from Changes tab

[osma]

As discussed in the review on Tuesday, we should make the following changes to the Changes tab implemented in #140:

• Remove the Modified concepts tab (and the whole subtab bar)
• Group the changes by month instead of by day

Also we should check that the ordering is consistent. It seems that at least in the Allärs listing (http://dev.finto.fi/allars/en/new?clang=sv) the dates suddenly jump from October to June.