Perspective view filtering, sorting and pagination on backend side

[vmonakhov]

Now PerspectiveView component gets all lexical entries for current perspective from database and processing the data on frontend side. It is not the optimal way in case of huge perspective (tens and hundreds of thousands entries). We need to process (filtering, sorting, pagination) the data on backend side and pass it page by page to frontend. In detail:

1. Filtering should have CaseSensitive and Regexp options. Probably, just created entries should not be filtered, should be visible any way.
2. Custom sorting goes by some field and can be ascending or descending. Default sorting is by 'Meaning' field and/or by client/object id from older to newer entries. Be careful, current perspective may not have 'Meaning' field. Just created entries should not be sorted by any field.
3. Before pagination you should separate (if any) old empty entries, just created empty entries, just created filled entries and count all the obtained entries. Just after this you can split data by pages.
4. Old empty entries should be visible in the beginning of the whole table in edit mode only. Newly created empty and filled entries should be visible in the beginning of any page over its limit without any custom filtering or sorting.

[vmonakhov]

All the listed above points are solved.

• Filtering and sorting are processed using sql requests.
• Filtering can be case sensitive or insensitive and/or using regexp.
• Sorting goes after filtering and uses its result.
• Sorting can be ascending or descending. In ascending case we choose lexicographically minimal entity within every lexical entry and sort our entries by them. We sort entities themselves within every lexical entry as well.
• Empty and/or newly created entries we show as required.

[vmonakhov]

Added sorting for 'Paradigm and contexts' field by entities amount (this field has empty contents). https://github.com/ispras/lingvodoc/commit/84b7a74aeb8bf6bff82bbd46c2f74a0088877864