Tiny issues collection

[w568w]

This thread collects recent issues from users (e.g. FDUHole).

• [ ] Optimization of rendering a post containing lots of images (FDUHole PID: 102609)
• [ ] Digital watermark, or something works in similar ways (FDUHole PID: 101949)
• [x] Higher contrast in unread/read messages (FDUHole PID: 97057)
• [x] More color themes. Maybe a customizable theming system (FDUHole PID: 85007)
• [x] Manual language settings (FDUHole PID: 93169)
• [x] Restrict the semester start date to a Monday, in case of mistaken selection.
• [ ] Support manually adding courses to the timetable (FDUHole PID: 64204)
• [x] Exam & Score page usually falls back to Data Center source when first entering the page, and then uses Edu Service source on the following entering. This situation even happens when user has got net access to Fudan Edu Service. We need to figure out why.