[1.7] Title and subtitle changes break old content

Verification builds: 1.7.0.2 R7 Beta 21
Summary: There have been changes in the hud_screen.json UI file in 1.7. Specifically in "hud_title_text." These changes are not backwards compatible and thus breaks existing content referencing "hud_title_text" and its children.

Impact: Two-fold. 1. An assert if the old variable is used and not the new "#text". 2. If the variable is changed to the new "#text" then the title just simply does not appear and does not give any errors.

Repro Steps: 

**Demo 1:

1. Download A Nightmare in Candyworld from the store.
2. Create the world with cheats on
3. Run /title @p title hi
4. Observe the assert.

Demo 2:

1. Create an unencrypted version of A Nightmare in Candyworld
2. Go to resource_packs->nicRP->ui->hud_screen.json
3. Change line 24 from ``"text": "$title_text",`` to ``"text": "#text"``
4. Change line 45 from ``"text": "$subtitle_text",`` to ``"text": "#text"``
5. Save the file
6. Enter the world with cheats enabled
7. /title @p title hi
8. Observe that no title appears.

Demo 3:

1. Create an unencrypted version of A Nightmare in Candyworld
2. Go to resource_packs->nicRP->ui->hud_screen.json
3. Replace that file with the new one attached to this bug report hud_screen.json (this file is identical except the the entire "hud_title_text" section has been replaced with the new one from the 1.7 ui file. I then made the intended changes which are changing the font size of "title" to "large" instead of "extra_large" and changing "subtitle" to "medium" instead of "large."
4. Save the file.
5. Open the world with cheats on.
6. /title @p title hi
7. Observe that everything works normally. This makes sense because this is the updated version of the UI format.

Observed Results:

**Titles cause asserts and/or do not work at all.

Expected Results:

Backwards compatibility with existing content.

Screenshots/Videos attached: No

Notes: I understand that UI will break in these new changes but I hope there is some possibility for retaining backwards compatibility. 

Regression Build: 1.6.0.8