Add character limit to Thumbnail URL

[calculuschild]

We've already had some people try to store whole Base64 images in the URL which massively increases storage size and bottlenecks loading and saving of brews, potentially leading to full crashes of the server. The current limit is 124 but due to the max size of Google Drive file properties (the Homebrewery will crash to an error page), but this is pretty restrictive and occurs commonly in standard URLs. Hows about... 256 characters for starters?