Friendlier error if filter is not found

If user sets, e.g., `filters [:name, :manager]`,
and manager is not set to `filterable true`,
RailsAdmin currently raises a NoMethodError
when trying to call `.type` on nil.

This change raises a more descriptive error
to help the user find the problem
Merge pull request #2079 from nathanl/friendlier_filter_error

Friendlier error if filter is not found
Move navigation out into partial so it can be easily overridden
Merge pull request #2081 from thefrontiergroup/feature/easily_replace_header

Move navigation out into partial so it can be easily overridden
Bump version to 0.6.4
Disallow font-awesome-rails versions >= 5
Disallow sass-rails versions >= 5
Don't package test files with gem
Bump version to 0.6.5
Disallow sass-rails versions >= 6
Support for specific locale on rails_admin panel
Fixed locale for model name and fields