ChangeTags: Rename "extension" to "software"

ChangeTags: Rename "extension" to "software"

Most end users don't care about whether a tag comes from MediaWiki core
or an extension. Since we now have tags that are added by core, let's
rename all of the "extension" terminology to the more generic
"software".

Not renamed in this patch is the i18n message "tags-source-extension",
which will be done in a separate patch to coordinate with TWN, and the
"source" output of the API, to avoid a breaking change.

Change-Id: Ic7b32b3e4bef0c0153a7cf302eef4aa07c52c9f2