refactor(external_link): migrate config during load_config

[SukkaW]

What does it do?

Instead of migrating old boolean format config into object format every time the filter is executed, run the config migration during load_config.

How to test

git clone -b config-external-link-process https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.002%) to 98.236% when pulling 10d8388 on SukkaW:config-external-link-process into 97cb698 on hexojs:master.

[curbengh]

This is a minor breaking change since config.external_link is modified. It was already a breaking change in #3675, and later attempted to retain original config.external_link #4134.

However, most probably users won't notice the difference, other than seeing the deprecation message; this only breaks if user has a script or theme that checks truthy value, i.e. if (config.external_link), it's now always truthy.