[pgAdmin4][Patch]: File Manager & Backform FileControl

Hi,

The patch is for *File Manager and Backform FileControl*.

*File Manager:* It allows user to store their files at one place. The user
can set path to this directory in preferences which file manger will use.

It will be used by various modules of pgAdmin4 such as
*import data from file, query tool, backup & recovery tool* so on.. It
allows user to perform various operations such as:

- File deletion
- File upload
- Create new directory
- File rename
- File download and
- List files & folders in list and grid view.

*FileControl: *This control is used with *File Manager. *It can be used for:

- File Selection
- Folder Selection
- File Creation
- Storage Manager Dialog.( it supports all operations listed above).

Usage*:*

schema:[{
id: 'select_file', label: '{{ _('Select File')}}',
type: 'text', *control*: *Backform.FileControl*,
*dialog_type: 'select_file'*, *supp_types: ['*', 'pdf', 'text', 'patch',
'py']*
}

Where:

- *control* - It should be *Backform.FileControl*
- *dialog_type *- the type of dialog to use.
- options can be: *select_file, select_folder, create_file &
storage_dialog*.
- *supp_types(*not mandatory) - the types of files allowed to show and
upload.

Please review the patch and let me know for the review comments.

Thanks
Surinder Kumar