fix: modify third nav title when renaming subfolder

[prestonlimlianjie]

This PR fixes #185.

Problem

The current renameSubfolder route method does the following:

• Rename all files from ${folder}/${previousSubfolder}/${fileName} to ${folder}/${newSubfolder}/${fileName} using git trees
• Modify the collection.yml file within the collection folder

However, this neglects to modify the third_nav_title fields within the collection subfolder files.

Fix

This PR fixes the abovementioned problem by taking the following approach:

• For each file in the ${folder}/${previousSubfolder} folder
  • Create the new content for each file by modifying the third_nav_title field in each retrieved file
  • Save the new content in the new ${folder}/${newSubfolder} folder
• Modify the collection.yml file within the collection folder

Note

This fix - while functional - is not ideal because it just adds more business logic to the routes layer. We will clean this up in the upcoming backend refactor.

[kwajiehao]

Lgtm

As discussed offline, we will need to revert the effects of this change when we eventually modify our templates to use collections.yml instead of each individual page's third_nav_title front matter attributed. Issue #202 was created to track this