Extremely optimize png images losslessly using zopflipng

[PeterDaveHello]

Description

Extremely optimize PNG images losslessly using Google's Zopflipng.

18 PNG images compressed as below, looks like saving about ~10% size in average:

 docs/stirling-pdf.png                                | Bin 50887 -> 45004 bytes
 docs/stirling.png                                    | Bin 8860 -> 8050 bytes
 images/login-dark.png                                | Bin 31740 -> 27493 bytes
 images/login-light.png                               | Bin 30897 -> 26998 bytes
 images/settings-light.png                            | Bin 44896 -> 36838 bytes
 images/stirling-home-dark.png                        | Bin 248109 -> 216213 bytes
 src/main/resources/static/android-chrome-192x192.png | Bin 4161 -> 3875 bytes
 src/main/resources/static/android-chrome-512x512.png | Bin 9419 -> 8590 bytes
 src/main/resources/static/apple-touch-icon.png       | Bin 3937 -> 3654 bytes
 src/main/resources/static/favicon-16x16.png          | Bin 829 -> 580 bytes
 src/main/resources/static/favicon-32x32.png          | Bin 1565 -> 1129 bytes
 src/main/resources/static/favicon.png                | Bin 8860 -> 8050 bytes
 src/main/resources/static/images/signature.png       | Bin 19683 -> 19631 bytes
 src/main/resources/static/mstile-144x144.png         | Bin 3368 -> 3196 bytes
 src/main/resources/static/mstile-150x150.png         | Bin 3312 -> 3095 bytes
 src/main/resources/static/mstile-310x150.png         | Bin 3504 -> 3281 bytes
 src/main/resources/static/mstile-310x310.png         | Bin 5947 -> 5424 bytes
 src/main/resources/static/mstile-70x70.png           | Bin 2733 -> 2536 bytes

Checklist

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have attached images of the change if it is UI based
• I have commented my code, particularly in hard-to-understand areas
• If my code has heavily changed functionality I have updated relevant docs on Stirling-PDFs doc repo
• My changes generate no new warnings
• I have read the section Add New Translation Tags (for new translation tags only)