Fix #24856
Rename "context.contextKey" to "context.WebContextKey", this context is
for web context only. But the Context itself is not renamed, otherwise
it would cause a lot of changes (if we really want to rename it, there
could be a separate PR).
The old test code doesn't really test, the "install page" gets broken
not only one time, so use new test code to make sure the "install page"
could work.
Replace #16455
Close #21803
Mixing different Gitea contexts together causes some problems:
1. Unable to respond proper content when error occurs, eg: Web should
respond HTML while API should respond JSON
2. Unclear dependency, eg: it's unclear when Context is used in
APIContext, which fields should be initialized, which methods are
necessary.
To make things clear, this PR introduces a Base context, it only
provides basic Req/Resp/Data features.
This PR mainly moves code. There are still many legacy problems and
TODOs in code, leave unrelated changes to future PRs.
The old code is unnecessarily complex, and has many misuses.
Old code "wraps" a lot, wrap wrap wrap, it's difficult to understand
which kind of handler is used.
The new code uses a general approach, we do not need to write all kinds
of handlers into the "wrapper", do not need to wrap them again and
again.
New code, there are only 2 concepts:
1. HandlerProvider: `func (h any) (handlerProvider func (next)
http.Handler)`, it can be used as middleware
2. Use HandlerProvider to get the final HandlerFunc, and use it for
`r.Get()`
And we can decouple the route package from context package (see the
TODO).
# FAQ
## Is `reflect` safe?
Yes, all handlers are checked during startup, see the `preCheckHandler`
comment. If any handler is wrong, developers could know it in the first
time.
## Does `reflect` affect performance?
No. https://github.com/go-gitea/gitea/pull/24080#discussion_r1164825901
1. This reflect code only runs for each web handler call, handler is far
more slower: 10ms-50ms
2. The reflect is pretty fast (comparing to other code): 0.000265ms
3. XORM has more reflect operations already
