Add default host for different database

[lunny]

When set DBType as postgres but left HOST empty, then the default value of host in installation page will become 127.0.0.1:3306. It's totally wrong.

[wxiaoguang]

I think it can be fixed in https://github.com/go-gitea/gitea/blob/main/web_src/js/features/install.js

Then we can have a unique behavior, instead of splitting code into backend and frontend.

[silverwind]

I think it can be fixed in https://github.com/go-gitea/gitea/blob/main/web_src/js/features/install.js

But that applies only to installation, what about already installed apps that just have this configuration wrong? I think certain duplication may be necessary.

[wxiaoguang]

For installed config, if the HOST is lost, then the user's config file must have been corrupted, we should not guess a potential incorrect config option for the user either. Doing too many uncertain things will cause more troubles.

[6543]

I think it should go in as is ... the UI part is another thing - witch should be fixed by this too ...

[wxiaoguang]

The UI is the key problem, the purpose of this PR is to make the install page show correct default database host, it can be fixed in JS clearly. If we do not want to fix the install UI, we do not need to patch anything.

Changing the default values of the setting module in backend may bring some negative effects:

1. It splits the default values from install page, and it makes it difficult to keep them the same in future (some developers may forget one of them).
2. This changed behavior is not documented. No one has the expectation that HOST has default values (and the default values differ by TYPE)
3. If we want to refactor the setting module in future, such default values may lead to breaking changes.

Since this PR was marked to wip by lunny, I think we can keep it as wip and wait for lunny's thoughts.

[techknowlogick]

marking as blocked per @wxiaoguang's feedback

[lunny]

replaced by #17919