Filtering doctors

[edyta555]

Added a filtering mechanism for doctors according to the following rules:

• capitalization does not matter
• we user enters many specializations, doctors who have all of them are searched
• a blank field is treated as no filtering (for example, when the field for a city is empty, doctors from all cities are searched)
• white spaces are ignored

Styling of the 'FilterDoctor' component will take place in the next step.