replaced deprecated utf8 functions

For now this uses full qualified namespaces, sensible imports may come
later.

added filter to extension listing in CLI component

fix color for noninstalled extensions

show disabled extensions in gray

warn about inaccessible repo api

bugfix: access check was never cached

Now the API ping result is cached for 10 minutes and can be refreshed
using the purge parameter as intended.

First go on a CLI component for the extension manager

This allows managing plugins from the command line which is useful in
automatic deployments (eg. through salt or ansible).

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Move list of plugin types to plugin controller constant

There is no need to have them in a global variable.

moved plugin controller to Extension namespace

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Rename HTTPClient namespace to HTTP

This should make namespace a bit more flexible in scope and allow us to
move more functionality there later.

Refactor HTTPClient into different files

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Refactor cache.php into different files

Add files via upload (#2679)

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Merge pull request #2577 from chang-zhao/patch-5

Add links "plugins" & "templates" to intro_search.txt

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translation update for zh:extensions, revert "offline" to "manual"

translation update for zh:extensions, closes #2572