Add morphology support to glossary and search

**Is your feature request related to a problem? Please describe.**
Base language of my project is Russian so my glossary is Russian>Other language
But some terms are not shown if they are in different case. As an example:
Our project frequently uses the word "карта" (map). So I made a glossary entry for term "карта".
If the source string contains this term in nominative case (карт**а**) the glossary shows it without problem. But if the source contains this word in genitive case (карт**ы**) the term is not shown in the glossary tab.
Although sometimes it works. For example glossary term "справочник" (reference) shows up if the source contains plural form "справочник**и**".
The same goes to the "Search" functions - it doesn't search for all the forms of a word.

**Describe the solution you'd like**
There is an open source spell checker [Hunspell](http://hunspell.github.io) which has a lot of dictionaries created for it (for example [here](https://github.com/wooorm/dictionaries/tree/master/dictionaries)) and it would be nice to implement it's morphology dictionaries into Weblate's glossary and search functions.

**Describe alternatives you've considered**
An alternative is to input all the word's forms into glossary, but it's a cumbersome task.