Fixing edit field problems (both visual and security) and other enhancements

Closes #369
Closes #408
Closes #467
Closes #696

<b>List of fixes and modifications:</b>
- Do not check (<a href="https://github.com/FrozenNode/Laravel-Administrator/blob/master/src/views/templates/edit.php#L15" target="_blank">here</a> & <a href="https://github.com/FrozenNode/Laravel-Administrator/blob/master/src/views/templates/settings.php#L5" target="_blank">here</a>) if a field is editable because we still want to display it's value in an 'uneditable' format
- Security: do not save a field into the model if it is not editable (see: #696) (also closes #369)
- Skip validating filters on uneditable fields as we don't want to save them anyway (see above)
- Fix bool values: if - for some reason - false (zero) was stored as a string in database, (string)"0" resulted in true (see #467)
- edit_fields "editable" option is now working right and displays value for fields: <br>
"image" or "datetime" (see #408) + "file", "color", "enum" ( '<i>Option Name (Value)</i>' displayed), "password" ('***' displayed for passwords)
- Hide "characters left" if a field is not editable (<a href="https://github.com/FrozenNode/Laravel-Administrator/blob/master/src/views/templates/edit.php#L24" target="_blank">here</a> & <a href="https://github.com/FrozenNode/Laravel-Administrator/blob/master/src/views/templates/edit.php#L35" target="_blank">here</a> & <a href="https://github.com/FrozenNode/Laravel-Administrator/blob/master/src/views/templates/settings.php#L10" target="_blank">here</a> & <a href="https://github.com/FrozenNode/Laravel-Administrator/blob/master/src/views/templates/settings.php#L21" target="_blank">here</a>)

<b>Added strings to language files (display these if a file/image is neither editable, nor uploaded):</b>

     'no_image_uploaded' => 'No image uploaded',
     'no_file_uploaded' => 'No file uploaded',