Minor fixes, added LinkedIn, made some variables optional

[poseen]

Changes

Header

• Optional social network links: when one is not set, then it won't appear.
• Added LinkedIn as an option. Can be defined in site.json
  Example:

  "social": {
    "github": "fooGitHub",,
    "twitter": "fooTwitter"
    "linkedin": "fooLinkedIn"
  },

Newsletter

• newsletter section in site.json is optional. If it wasn't present, build has failed previously.
• Newsletter twitter link has an extra visibility condition: siteConfig.social.twitter has to be present.

Testing

Tried the following cases manually:

Header

• Unset social.twitter --> twitter link disappeared.
• Set social.linkedin --> LinkedIn link appeared

Newsletter

• Removing newsletter --> build succeeds, subscription box not visible, twitter link not visible.
• Readding newsletter, setting "showTwitterBanner": true, but unsetting social.twitter --> Twitter footer is not visible.
• Readding newsletter, setting "showTwitterBanner": true, setting social.twitter --> Twitter footer is visible.
• Enabling newsletter subscription box by setting newsletter.revueUsername --> Subscription box is visible, twitter footer is invisible.