Sanitize content to avoid Formula Injections on CSV export

During the export in CSV, user content should be sanitized properly by adding a 'character if the cell is starting with one of the following characters: =, +, - or @ .

How to reproduce:

• create a board
• add a column title or a note with: =1+1
• export to CSV
• open it in a spreadsheet, the formula is interpreted in the spreadsheet

Note: we can think that people will be able to notice malicious content when they see the board, but a attacker can "trick" users by masking suspicious content under other notes.

Reference:

https://www.contextis.com/resources/blog/comma-separated-vulnerabilities/