compose_paste: Convert large pasted text into uploaded text file.

When pasting a long piece of text into the compose box (or
message edit textarea), we show a banner giving an option to
put the text into a file and upload it.

The banner is only shown if the text is larger than
`MINIMUM_PASTE_SIZE_FOR_FILE_TREATMENT`.

If the user chooses to "convert to file", the textarea content is
restored to its state before pasting and a file with the pasted
content is uploaded. The banner is hidden as soon as any change is
made to the textarea content to avoid inconsistent state and
confusion.

Fixes #33107.

Co-authored-by: Aditya Chaudhary <aditya.chaudhary1558@gmail.com>

tools/lib/capitalization.py

@@ -80,6 +80,8 @@ IGNORED_PHRASES = [
     r"email",
     r"enabled",
     r"signups",
+    # Pasted text filename
+    r"PastedText",
     # Placeholders
     r"keyword",
     r"streamname",