Bump nuxt from 2.14.3 to 2.15.8 in /frontend

Sourced from nuxt's releases.

<h2>v2.15.8</h2> <h3>🐛 Bug Fixes</h3>

• vue-app
  • #9460 Don't normalise route path if it's valid
• babel
  • #9631 Loose option for babel private-property-in-object

<h2>v2.15.7</h2> <h3>🔰 Security advisory</h3>

Please upgrade to nuxt@^2.15.7 if using nuxt@2.15.5 or nuxt@2.15.6

<h3>🐛 Bug Fixes</h3>

• vue-app
  • #9431 Check whether a route exists within the nuxt app before replacing

<h2>v2.15.6</h2> <h3>🐛 Bug Fixes</h3>

• types
  • #9270 Pin to @types/webpack-dev-middleware@4.1.2 (resolves #9268)

<h2>v2.15.5</h2> <h3>🐛 Bug Fixes</h3>

• babel
  • #9232 Loose option for babel class-properties and private-methods (resolves #9224)
• vue-app
  • #9201 Use route.replace instead of router.push to trigger navigation guards (resolves #9111)
• builder
  • #9153 Resolve aliases in build.watch paths (resolves #9045)
• cli
  • #9152 Add warning for css-loader < 4.2 (resolves #9117)

<h3>💖 Thanks to</h3>

• Thomas Beduneau (@​enwin)
• @​rlam3

<h2>v2.15.4</h2> <h3>🐛 Bug Fixes</h3>

• vue-app
  • #8978 Reload page once after loading chunk error (resolves #3389)
  • #9008 Fallback to global nuxt instance of $root is not available (resolves #8995)

... (truncated)

• d4b9e4b v2.15.8
• af1d3e9 chore: skip audit due to outdated lockfile
• 1ccd151 fix(vue-app): don't normalise route path if it's valid (#9460)
• af49199 fix(babel): loose option for babel private-property-in-object (#9631)
• 4f370e2 v2.15.7
• 85615a5 fix(vue-app): check whether route exists within nuxt app before replacing (#9...
• 0eae970 chore(release): v2.15.6
• fc5fb58 hotfix: revert feat additions
• cd1fc62 chore(deps): update ufo to 0.7.4
• a80ddee chore(deps): update all non-major dependencies (#9281)
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