Revise the way the 'most recently used' character picker works.

[70ray]

Although the present system works it has some shortcomings:

• The data is stored on the users device, so if a user uses more than one device it does not transfer between them.
• The data remains on the device for ever and thus gradually increases in size as the user moves onto new projects. (It can be removed by using the developer tools.)

One way to address these issues would be to store the data on the server. It could be uploaded when the user saves a page and downloaded with the other character pickers. It could be removed by a crontab process when the project leaves the rounds. This could be done in conjunction with the new proofreading interface.