Explicit redirect handling

[AMDmi3]

To be more consistent and informative, and also to obey RPS limits more strictly, redirects are needed to be handled explicitly. E.g.

• aiohttp's automatic redirect following should be disabled
• on redirect, the target needs to be added to the links table if it's not yet there and checked as a normal link (need to think of last_seen propagation and clever next_check handling, so redirect chains are usually checked at once).
• repology /link/ viewer should be modified accordingly
• there should be some sort of bad status propagation from target to redirect source (with loop detection) (or should we do in on the fly?)
• could add id column to links table,

This would allow to handle http->ftp redirects btw.