[General] File upload

In order to upload files, the user needs to click on upload button. A popup appears, where the user needs to drag and drop files to be uploaded.

Files added can be: PDF, DOCX, DOC, JPG, JPEG, PNG.
Maximum file size: 5MB
The documents will appear in the popup as a list, containing:
- File name
- (uploading) progress bar
- File size
- X (delete button)


If a file could not be uploaded, error message shows: “Documentul nu a putut fi încărcat”, “Documentul are peste 5MB”

After uploading, the attachments will be shown as a list, each item in the list having
- icon
- file name
- X button (delete button)

Example: attaching document to an entry in the Balance sheet table ([issue #13](https://github.com/code4romania/alex-api/issues/13))

Example mockup link
https://www.figma.com/file/W7ppl5xGFpB3atVWwYL7qN/Alex-(asistentul-bugetar-al-ONG-urilor)?node-id=1139%3A24529
https://www.figma.com/file/W7ppl5xGFpB3atVWwYL7qN/Alex-(asistentul-bugetar-al-ONG-urilor)?node-id=1139%3A24428
https://www.figma.com/file/W7ppl5xGFpB3atVWwYL7qN/Alex-(asistentul-bugetar-al-ONG-urilor)?node-id=1139%3A24438

In order to attach more files, the user needs to click the upload files button and repeat the process.