ReadText should linearize text to preserve indexes

[jlewi]

Right now ReadText linearizes the text just by concatenating strings as it reads through elements.

p22h/backend/pkg/gdocs/text.go

Line 10 in c9902c8

func ReadText(doc *docs.Document) (string, error) {

As a result, the order in which elements are processed will affect the position of the text in the resulting string. The indexes may therefore not match the indexes used in the Google Document
https://developers.google.com/docs/api/reference/rest/v1/documents#ParagraphElement

This will make it impossible to map any annotations on the text back to google doc elements.

We should linearize text in order to align with Google Documents indexing. This should be straightforward. As we read elements we can populate a string using the indexes specified in the Google Doc element.