Fix SVG icon and replace Bootstrap logo with PyLadiesCon logo

[samikshakhadka]

1. Added the missing SVG symbol definition for the arrow icon in the Sign Up button at the top of the base template
2. Set the arrow icon's fill color to white for better visibility against the button background

I've left the Bootstrap logo in the welcome page unchanged for now, as I wasn't sure whether we want to:

1. Remove it entirely
2. Replace it with the PyLadiesCon logo

I can update this PR once we have guidance on the preferred approach for the logo.
Let me know if you'd like me to make any adjustments to the current changes. @Mr-Sunglasses @Mariatta #79

[Mr-Sunglasses]

Thanks @samikshakhadka, requesting @Mariatta for a final review.

[Mariatta]

Thanks for the PR. I'm merging your other PR instead of this one. (#104)