luci-app-lxc: fix configure lxc container POST request

[ducloc464]

• This PR is not from my main or master branch 💩, but a separate branch ✅
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• Incremented 🆙 any PKG_VERSION in the Makefile
• Tested on: (x86_64, 23.0.5, Chrome) ✅
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• ( Optional ) Closes: e.g. luci-app-lxc: "Action failed!" when configuring container #6098
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• Description: the code for sending POST request to Luci was in wrong format, It did not have key value for formvalue and the value has to decode from encodeURIComponent

The backend code is:
local lxc_configuration = luci.http.formvalue("lxc_conf")
but frontend do not send formvalue with key "lxc_conf":
new XHR().post('<%=luci.dispatcher.build_url("admin", "services")%>/lxc_configuration_set/' + lxc_name, "lxc_conf=" + encodeURIComponent(lxc_conf)

The fix is:

new XHR().post('<%=luci.dispatcher.build_url("admin", "services")%>/lxc_configuration_set/' + lxc_name, {"lxc_conf": encodeURIComponent(lxc_conf)}

and decoded string is required for LXC

lxc_configuration = luci.http.urldecode(lxc_configuration, true)