Fix serving of raw wiki files other than .md (#5814)

* Fix serving of raw wiki files other than .md

Closes #4690.
Closes #4395.

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

* Simplify code at routers/repo/wiki.go

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

* Add more files to user2/repo1.wiki for testing

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

* Update macaron to v1.3.2

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

* Add tests for WikiRaw

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

* Fix NewResponseWriter usage due to macaron update

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

* Add raw to reserved wiki names

Signed-off-by: Gabriel Silva Simões <simoes.sgabriel@gmail.com>

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