Replace Whitney font with Lato throughout app

[albullington]

We're making a change to replace Whitney fonts with Lato throughout iNaturalist. The full list of fonts we need to replace are:

• [x] Book Italic
• [x] Book Pro
• [x] Book
• [x] Book Italic Pro
• [x] Light Italic
• [x] Light Pro
• [x] Light
• [x] Light Italic Pro
• [x] Medium Italic
• [x] Medium Pro
• [x] Medium
• [x] Medium Italic
• [x] Semibold Italic
• [x] Semibold Pro
• [x] Semibold
• [x] Semibold Italic Pro

To complete this issue, we should:

• [x] Remove Whitney fonts
• [x] Add Leto fonts
• [x] Run npx react-native-asset to link new fonts
• [x] Make sure ios/Info.plist includes Leto fonts, not Whitney fonts (no need to include the -Pro files in Info.plist since those are for Android-only
• [x] Update styledComponents.js with Leto fonts
• [x] Update tailwind.config.js with Leto fonts
• [x] Audit the app and replace Whitney wherever we added a custom fontFamily (this should be SearchBar, UserText, Button, TextInputSheet, Wikipedia, CustomDrawerContent, and navigationOptions). It's probably ideal to pull these custom font families out into a file of constants so we can have a single source of truth instead of updating this across multiple files. (For example, a file that contains a variable:

const DEFAULT_FONT_FAMILY = `Leto-Light${Platform.OS === "ios"
      ? ""
      : "-Pro"}

[albullington]

@abhasinat can you confirm these are the correct replacements?

1. Whitney Light -> Lato Light
2. Whitney Book -> Lato Regular
3. Whitney Medium -> Lato Bold
4. Whitney Semibold -> Lato Black

[abhasinat]

Here are the correct replacements:

• Whitney Book -> Lato Regular
• Whitney Medium -> Lato Medium
• Whitney Semibold -> Lato Bold

I had earlier also written "Whitney Light -> Lato Light" but I don't think I used Whitney Light in the app at all.