The old `HTMLRender` is not ideal.
1. It shouldn't be initialized multiple times, it consumes a lot of
memory and is slow.
2. It shouldn't depend on short-lived requests, the `WatchLocalChanges`
needs a long-running context.
3. It doesn't make sense to use FuncsMap slice.
HTMLRender was designed to only work for GItea's specialized 400+
templates, so it's good to make it a global shared instance.
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