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This is now up to the user to chose how many items they chose to fetch
from the history, through the returned Query's item() method

It's now using the regular Media table & requests

This will ease up the migrations, as those are the only tables we
absolutely need to keep the content of. It will be easier to just create
a new version of the model, and include it from the migration script,
instead of having the main "create table" and the migration one

We now return all media as a single query

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

All request that could return more than one row (ie. all member function
that used to return an std::vector<std::shared_ptr<T>> will now return a std::unique_ptr<Query<T>>
The query object can return:
- The amount of row represented by that query (->count())
- A subset of the rows (->items(count, offset))
- All rows (->all())

Instead of doing it after fetching. This will ease up pagination

So we can use File.is_present and to simplify integration of Sorting
for search results

This has the extra benefits of reducing our use of Media.is_present

Fix #34

To help when a const reference is needed as the return value.

This will be used by all classes using the Thumbnail class